Inkjet printing mechanisms may be used in a variety of different printing devices, such as plotters, facsimile machines and inkjet printers, collectively referred to herein as printers. These printing mechanisms typically use a printhead to shoot drops of ink onto a page or sheet of print media. Some inkjet print mechanisms utilize a type of printhead called a cartridge that carries a self contained ink supply back and forth across the media. In the case of a multi-color cartridge, several printheads and reservoirs may be combined into a single unit, with each reservoir/pen combination for a given color being referred to herein as a “pen.”
Other inkjet print mechanisms, known as “off-axis” systems, propel only a small amount of ink in the printhead across the media, and include a main ink supply in a separate reservoir, which is located “off-axis” from the path of printhead travel. Typically, a flexible conduit or tubing is used to convey the ink from the reservoir to the printhead. In these types of print mechanisms the printhead itself is referred to as a “pen”. A pen may also have a cap or capping mechanism such that when the pen is not printing, the pen is covered. This may serve to prevent the pen from drying and/or to otherwise protect the pen from the environment.
Each pen includes very small nozzles through which the ink drops are fired. The particular ink ejection mechanism within the pen may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal pen technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett Packard Company. In a thermal ejection system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor.
To print an image, the pen is scanned back and forth across above the media in an area known as a print zone, with the pen shooting drops of ink as it moves. By selectively energizing the resistors as the pen moves across the media, the ink is expelled in a pattern on the media to form a desired image (e.g., picture, chart or text). The nozzles are typically arranged in one or more linear arrays. If more than one linear array is utilized, the linear arrays may be located side-by-side on the pen, parallel to one another, and substantially perpendicular to the scanning direction. As such, the length of the nozzle arrays defines a print swath or band. That is, if all the nozzles of one array were continually fired as the pen made one complete traverse through the print zone, a band or swath of ink would appear on the sheet. The height of this band is known as the “swath height” of the pen, the maximum pattern of ink which can be laid down in a single pass.
The orifice plate of the pen tends to accumulate contaminants, such as paper dust, and the like, during the printing process. Such contaminants may adhere to the orifice plate for various reasons including the presence of ink on the pen, or because of electrostatic charges that may build up during operation. In addition, excess dried ink may accumulate around the pen. The accumulation of ink or other contaminants may impair the quality of the output by interfering with the proper application of ink to the printing medium. Also, if color pens are used, each pen may have different nozzles which each expel different colors. If ink accumulates on the orifice plate, a mixing of different colored inks, known as cross-contamination, can result during use. If colors are mixed on the orifice plate, the quality of the resulting printed product can be affected. Furthermore, the nozzles of an ink-jet printer can clog, particularly if the pens are left uncapped for a period of time. For these reasons, it is desirable to service the pen by clearing the pen orifice plate of such contaminants and ink on a routine basis to prevent the build up thereof. This may be accomplished by a service procedure where a pen expels ink, is brought in contact with a wiper and expels ink again, also called a spit, wipe spit procedure. In some printers this service procedure is performed at the end of a print job based on certain criteria, for example, the number of drops fired since the last spit, wipe, spit procedure, the time a pen has been uncapped, upon a user request, when power has first been applied to the printer, etc. Service procedures such as the spit, wipe, spit procedure are desirable to maintain print quality but also contribute to increased print time because of the time required to perform the procedure and shorter pen life because wiping over time may degrade the nozzle plate by scratching and distorting the surface.
U.S. Pat. No. 5,455,608 describes how a printer may schedule service on a pen solely based on the result of a drop detection step. Before starting a plot the printer performs a drop detection on all pens present to detect if any nozzles are non-firing, also referred to as a “nozzle out” condition. If a nozzle out condition is detected in a pen, the printer triggers an automatic recovery servicing process for servicing the malfunctioning pen to clear or otherwise recover the malfunctioning nozzle.
This process includes a sequence of nozzle recovery or clearing procedures of increasing severity. At the end of each procedure a new drop detection test is performed on the pen, to detect if the pen is fully recovered. If the drop detection test indicates that a nozzle out condition continues to exist, another servicing procedure is performed. If, after a predetermined number of procedures, the pen is still not fully recovered (i.e. at least one nozzle is still out) the user is instructed to replace the pen or to discontinue the current nozzle check. Thus, a “nozzle health” detection is performed before each print job and recovery procedures are performed based on a fixed threshold, in this example, at least one nozzle remaining non-firing.
One disadvantage of this particular process is that if the printer is not able to fully recover the failing nozzles, some nozzles are unstable, or the system is unable to compensate for the failing nozzles using error hiding techniques, the system may recognize that the pen is not fully recovered and may run the recovery servicing process at various times, for example, at the beginning of each print job, when the nozzle health indicates that the service process is required, or upon a user request. The system may run the recovery process until the pen has been fully recovered or replaced. This may lead to an unacceptable loss of throughput and a loss of printer productivity because the automatic recovery process is very time consuming, the recovery process consumes a large quantity of ink, particularly when running a priming function included in the recovery process, and before each plot, the printer directs the user to replace the pen or to discontinue the current nozzle check.
Another disadvantage of this process is that the pen is designated as either “able to print” or “unable to print” solely based on the number of nozzles either working or not working.